fix template default parameter for secondaryview

Framework/ProcessSequence/testProcessSequence.cc

@@ -147,7 +147,7 @@ public:
 class Decay1 : public DecayProcess<Decay1> {
 
 public:
-  Decay1(const int v) {
+  Decay1(const int) {
     cout << "Decay1()" << endl;
     globalCount++;
   }

Framework/StackInterface/SecondaryView.h

@@ -17,6 +17,10 @@
 
 namespace corsika::stack {
 
+  // forward-decl:
+  template <class T1, template <class> class T2>
+  class DefaultSecondaryProducer;
+
   /**
    * @class SecondaryView
    *
@@ -57,7 +61,7 @@ namespace corsika::stack {
 
   template <typename StackDataType,
             template <typename> typename ParticleInterface,
-            template <class T1, template <class> class T2> class MSecondaryProducer>
+            template <class T1, template <class> class T2> class MSecondaryProducer = DefaultSecondaryProducer>
 
   class SecondaryView : public Stack<StackDataType&, ParticleInterface>,
                         public MSecondaryProducer<StackDataType, ParticleInterface> {